Modern Views on Chemical and Biological Evolution

According to this theory life originated on early earth through physico-chemical processes of atoms combining to form molecules, molecules in turn reacting to produce inorganic and organic compounds.

Organic compounds interacting to produce all types of macromolecules which organised to form the first living system or cells.

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Thus according to this theory ‘life’ originated upon our earth spontaneously from non-living matter. First inorganic compounds and then organic compounds were formed in accordance with ever-changing environmental conditions. This is called chemical evolution which cannot occur under present environmental conditions upon earth. Conditions suitable for origin of life existed only upon primitive earth.

Oparin-Haldane theory is also called chemical theory or naturalistic theory

Modern views regarding the origin of life include chemical evolution and biological evolution:

1. Chemical Evolution:

(i) The Atomic Phase:

Early earth bad innumerable free atoms of ail those elements (e.g., hydrogen, oxygen, carbon, nitrogen, sulphur, phosphorus, etc.) which are essential for the forma­tion of protoplasm. Atoms were segregated in three concentric masses according to their weights, (a) The heaviest atoms of iron, nickel, copper, etc. were found in the centre of the earth, (b) Medium weight atoms of sodium, potassium, silicon, magnesium, aluminium, phosphorus, chlo­rine, fluorine, sulphur, etc. were collected in the core of the earth, (c) The lightest atoms of nitrogen, hydrogen, oxygen, carbon etc., formed the primitive atmosphere.

(ii) Origin of Molecules and Simple Inorganic Compounds:

Free atoms combined to form molecules and simple inorganic compounds. Hydrogen atoms were most numerous and most reactive in primitive atmosphere. First hydrogen atoms combined with all oxygen atoms to form water and leaving no free oxygen. Thus primitive atmosphere was reducing atmosphere (without free oxygen) unlike the present oxidising atmosphere (with free oxygen). Hydrogen atoms also combined with nitrogen, forming ammonia (NH3). So water and ammonia were probably the first compound molecules of primitive earth.

(iii) Origin of Simple Organic Compounds (Monomers):

The primitive atmosphere con­tained gases like CO2, CO, N, H2, etc. The nitrogen and carbon of the atmosphere combined with metallic atoms, forming nitrides and carbides. Water vapour and metallic carbides reacted to form the first organic compound, methane (CH4). Later on hydrogen cyanide (HCN) was formed.

Torrential rains must have fallen:

As the water rushed down, it must have dissolved away and carried with it salts and minerals, and ultimately accumulated in the form of oceans. Thus ancient oceanic water contained large amounts of dissolved NH3, CH4, HCN, nitrides, carbides, various gases and elements.

The early compounds interacted and produced simple organic compounds such as simple sugars (e.g., ribose, deoxyribose, glucose, etc.), nitrogenous bases (e.g., purines, pyrimidines), amino acids, glycerol, fatty acids, etc. Some external sources must have been acting on the mixture for reactions. These external sources might be (i) solar radiations such as ultra-violet light, X-rays, etc., (ii) energy from electrical discharges like lightning, (iii) high energy radiations are other sources of energies (probably unstable isotopes on the primitive earth). There was no ozone layer in the atmosphere.

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The oceanic water rich in mixture of organic compounds was termed by J.B.S. Haldane (1920) as ‘hot dilute soup of organic substances’. The ‘hot dilute soup’ is also called prebiotic soup’. Thus the stage was set for combination of various chemical elements. Once formed, the organic molecules accumulated in water because their degradation was extremely slow in the absence of any life or enzyme catalysts.

Experimental Evidence for Abiogenic Molecular Evolution of Life:

Stanley Miller in 1953, demonstrated it clearly that ultra-violet radiation or electrical discharges or heat or a combination of these can produce complex organic compounds from a mixture of methane, ammonia, water (stream of water), and hydrogen.

Miller circulated four gases— methane, ammonia, hydrogen and water vapour in an air tight apparatus and passed electrical discharges from electrodes at 800°C. He passed the mixture through a condenser. He circulated the gases continuously in this way for one week and then analysed the chemical composition of the liquid inside the apparatus. He found a large number of simple organic compounds including some amino acids such as alanine, glycine and aspartic acid. Miller proved that organic compounds were basis of life.

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Other substances, such as urea, hydrogen cyanide, lactic acid and acetic acid were also present. In another experiment Miller circulated the mixture of the gases in the same way but he did not pass the electric discharge. He could not get the significant yield of the organic compounds. Later on many investigators have synthesized a great variety of organic compounds including purines, pyrimidines and simple sugars, etc. It is considered that the essential ‘building blocks’ such as nucleotides, amino acids, etc. of living organisms could thus have formed on the primitive earth.

(iv) Origin of Complex Organic Compounds (Polymers):

A variety of amino acids, fatty acids, hydrocarbons, purines and pyrimidine bases, simple sugars and other organic compounds accumulated in the ancient seas. In the primeval atmosphere electrical discharge, lightning, solar energy, ATP and polyphosphates might have provided the source of energy for polymerisation reactions of organic synthesis. S.W. Fox demonstrated that if a nearly dry mixture of amino acids is heated, polypeptide molecules are synthesized.

Similarly simple sugars could form polysac­charides and fatty acids could combine to produce fats. Amino acids could form proteins, when other factors were involved. Thus the small simple organic molecules combined to form large complex organic molecules, e.g., amino acid units joined to form polypeptides and proteins, simple sugar units combined to form polysaccharides, fatty acids and glycerol united to form fats, sug­ars, nitrogenous bases, and phosphates combined into nucleotides which polymerized into nucleic acids in the ancient oceans.

2. Biological Evolution:

For origin of life, atleast three conditions are needed:

(a) There must have been a supply of replicators, i.e., self-producing molecules.

(b) Copying of these replicators must have been subject to error through mutation.

(c) The system of replicators must have required a continuous supply of free energy and partial isolation from the general environment.

The high temperature in early earth would have fulfilled the requirement of mutation.

Origin of Prebiotic Molecules:

The third condition, partial isolation, has been attained within aggregates of artificially formed prebiotic molecules. These aggregates are called protobionts which can separate combinations of molecules from the surroundings. They maintain an internal environment but are unable to reproduce. Two important protobionts are coacervates and microspheres.

Coacervates:

Oparin (1924) observed that if a mixture of a large protein and a polysaccharide is shaken, coacervates are formed. The coacervates contain mainly proteins, polysaccharides and some water. Oparin’s coacervates also show a simple form of metabolism. As these coacervates do not have lipid outer membranes hence they cannot reproduce. Thus they do not fulfill the requirement for probable precursors of life.

Microspheres:

When mixtures of artificially produced organic compounds are mixed with cool water microspheres are formed. If the mixture contains lipids, the surface of the microspheres consists of a lipid bilayer, reminiscent (remembering things of past) to the lipid bilayer of cell membranes. Sydney Fox (1950) heated a mixture of 18 ammo acids to temperatures of 130 to 180°C. He obtained stable, protein like macromolecules which he named protenoids.

When the protenoid material was cooled and examined under microscope, Fox observed small spherical cell-like units that had arisen from aggregations of protenoids. These molecular aggregates were called protenoid miciospheies. The first non-cellular forms of life could have originated 3 billion years back. They would have been giant molecules (RNA, Proteins, and Polysaccharides etc.).

Physical properties of protenoid microspheres:

They were spherical microscopic with about 1 to 2 (am in diameter, similar to the size and shape of coccoid bacteria.

Structural properties of protenoid microspheres:

Under electron microscope, concentric double layered boundaries around them have been observed through which diffusion of material occurs. They have the ability of motility, growth, binary fission into two particles and a capacity of reproduction by budding and fragmentation. Superficially, their budding resembles with those of bacteria and fungi.

Enzyme-like activities of protenoid microspheres:

They have been found to have catalytic activity, such as degradation of glucose. This enzymatic activity of protenoid microspheres is partially lost during heating.

The main drawback in the protenoid microspheres is that they have limited diversity. Thus the mechanism of partial isolation leading to the origin of protobionts still remains unsolved.

Since both protein and nucleic acids (together with other simpler substances) are required to develop and reproduce organisms living today, an obvious question is which of these substances arose first? No clear answer is available to it.

RNA First Model:

In recent years, evidences are in favour of RNA to be material of first formed gene (Woese, 1967, Crick 1968, Orgel 1973, 1986 Watson et al 1986, Darnell et al 1986). Thus RNA may have been the first polymer and some form of reverse transcription may have given rise to DNA and RBA and DNA started controlling protein synthesis.

Why RNA and not DNA was the first living molecule?

Enzymatic activities of RNA molecules are constantly being discovered, but no enzymatic activity has ever been attributed to DNA. Further, ribose is much more readily synthesized than deoxyribose under stimulated prebiotic conditions. A selective advantageous RNA molecule would be one that directs the synthesis of protein that accelerates the replication of particular RNA (i.e., RNA polymerase).

RNAs could have catalysed the formation of lipid like molecules that could have, in turn, formed plasma membrane and proteins. The proteins might have taken over most enzymatic functions because they are better catalysts than RNAs. If the first cells used RNA as their hereditary molecule then DNA evolved from an RNA template. Once cells evolved, DNA probably replaced RNA in most organisms.

Formation of the Earliest Cells:

(i) The first living organisms originated among organic molecules and in oxygen free atmosphere (reducing atmosphere). They presumably obtained energy by the fermentation of some of these organic molecules. They were anaerobes, capable of respiration in the absence of oxygen. They depended on the existing organic molecules for their nutrition and thus they were heterotrophs.

(ii) When the supply of existing organic molecules was exhausted, some of the heterotrophs might have evolved into autotrophs. These organisms were capable of producing their own or­ganic molecules by chemosynthesis or photosynthesis.

(a) Chemosynthesis:

The organisms performing chemosynthesis are called chemoautotrophs. They were anaerobic. Chemoautotrophs developed the ability to synthesise organic molecules from inorganic raw materials. Such a mode of nutrition is present even now in some bacteria, e.g., sulphur bacteria, iron bacteria, nitrifying bacteria.

(b) Photosynthesis:

The photosynthetic organisms, the photoautotrophs, developed the pig­ment chlorophyll by combination of simple chemicals. They prepared organic food by using solar energy captured with the help of chlorophyll. They lacked the biochemical pathways to produce oxygen. They were still anaerobic and utilized hydrogen from sources other than water.

At later stage, oxygen releasing photosynthetic organisms developed. These were similar to the existing blue green algae (cyanobacteria). They used water to get hydrogen and released oxygen. Addition of O2 to the atmosphere started oxidising the methane and ammonia, which began to disappear.

CH4 + 2O2 → CO2 + 2H2O

4NH3 + 3O2→ 2N2 + 6H2O

Life was present on Earth about 3.9 billion years ago. However, the oldest microfossils discovered so far are that of photosynthetic cyanobacteria that appeared 3.3 to 3.5 billion years ago.

Formation of Ozone Layer:

As oxygen accumulated in the atmosphere, the ultraviolet light changed some tifoxygen into ozone.

2O2 + O2 →2O3

The ozone formed a layer in the atmosphere, blocking the ultraviolet light and leaving the visible light as the main source of energy.

Origin of Eukaryotic Cells (True nuclear cells):

Aerobic respiration evolved sufficient oxygen in the primitive atmosphere. The prokaryotes gradually modified to adapt themselves according to new conditions. They developed a true nucleus and other specialized cell organelles. Thus free-living eukaryotic cell-like organisms originated in the Ancierit Ocean presumably about 1.5 billion years ago. Primitive eukaryotes led to the evolution of protists, plants, fungi and animals.

Summary of main steps in the origin of life according to modern theory of origin of life.